The regulation of blood glucose levels by insulin is achieved mainly by increased glucose transport exclusively into adipose and skeletal muscle tissue; De Fronzo et al. (1981) Diabetes 30:1000–1007 and James et al. (1985) Am. J. Physiol. 248:E567–E574. These are the only two tissues that express a specific isoform of the glucose transporter, GLUT4, which mediates the hormonal effect of insulin (for reviews of glucose transporter isoforms and their expression, see Deveskar and Mueckler (1992) Pediatr. Res. 31:1–13; Bell et al. (1993) J. Biol. Chem. 268:3352–3356; and Baldwin (1993) Biochim. Biophys. Acta 1154:17–49). The mechanism of glucose transport activation by insulin is the hormone-dependent enhancement of the rate of GLUT4 translocation from intracellular storage vesicles to the plasma membrane in such a way that the concentration of the transporter on the cell surface increases 10- to 40-fold, depending on the cell type and method of measurement (Zorzono et al. (1989) J. Biol. Chem. 264:12358–12363; Holman et al. (1990) J. Biol. Chem. 265:18172–18179; Slot et al. (1991) J. Biol. Chem. 113:123–135; Slot et al. (1991) Proc. Nat'l. Acad. Sci. USA 88:7815–7819; and Smith et al. (1991) Proc. Nat'l. Acad. Sci. USA 88:6893–6897). Glucose uptake is increased proportionally to the increment of GLUT4 molecules in the plasma membrane, suggesting that redistribution of transporters is the main, if not only, mechanism that accounts for this effect, Kandror and Pilch (1994) Proc. Nat'l. Acad. Sca USA 91:8017–8021.
It is believed that GLUT4 recycles in cells as a constituent of tissue-specific secretory-like microsomal structures, known as “GLUT4-containing vesicles”. In addition to GLUT 4, these vesicles have also been determined to include phosphatidylinositol 4-kinase, Del Vecchio and Pilch (1991) J. Biol. Chem. 266:13278–13283; vesicle-associated membrane proteins (“VAMPS”), Cain et al. (1992) J. Biol. Chem. 267:11681–11684; secretory component-associated membrane proteins (“SCAMPS”), Thoidis et al. (1993) J. Biol. Chem. 268:11691–11696; and Laurie et al. (1993) J. Biol. Chem. 268:19110–19117; and low molecular weight GTP-binding proteins of the Rab family, Cormont et al. (1993) J. Biol. Chem. 268:19491–19497. In addition to the proteins enumerated above, a novel zinc-dependent protease named insulin-responsive aminopeptidase (“IRAP”) has been identified and characterized as an important component of GLUT4-containing vesicles (designated previously as gp160, Kandror and Pilch (1994) Proc. Nat'l. Acad. Sca USA 91:8017–8021; Kandror et al. (1994) J. Biol. Chem. 269:30777–30780; and vp165, Keller et al. (1995) J. Biol. Chem. 270:23612–23618. Structurally, IRAP contains a 109-amino acid amino-terminal end which projects into the cytoplasm, a single 22-amino acid transmembrane domain, and a large catalytic domain within the lumen of the vesicle which is responsible for the protein's enzymatic activity, Keller et al. (1995) J. Biol. Chem. 270:23612–23618. In the basal state, IRAP is primarily located intracellularly, like GLUT4, but is markedly translocated to the cell surface in response to insulin, Mastick et al. (1994) J. Biol. Chem. 269:6089–6092; Kandror and Pilch (1994) Proc. Nat'l. Acad. Sca USA 91:8017–8021; Ross et al. (1996) J. Biol. Chem. 271:3328–3332; and Ross et al. (1997) Biochem. Biophys. Res. Commun. (1997) 239:247–251. Furthermore, it has been suggested that the amino terminus of IRAP, which contains two dileucine motifs and several acidic regions similar to those that occur in GLUT4, functions in the regulation of intracellular trafficking and retention of GLUT4; Waters et al. (1997) J. Biol. Chem. 272:23323–23327.
Insulin-responsive glucose transport is essential to the normal functioning and metabolism of fat and muscle tissue in normal animals (e.g., in normal human subjects). Insulin resistance of, for example, skeletal muscle glucose transport is a key defect in the development of impaired glucose tolerance (IGT) and type II diabetes. A more detailed understanding of the molecular mechanisms responsible for insulin-responsive glucose transport would greatly facilitate the development of therapeutic strategies aimed modulating (e.g., increasing) insulin responsiveness and ultimately treating subjects exhibiting IGT and/or having type II diabetes. In particular, the intracellular molecules involved in insulin-responsive glucose transport serve as useful target for modulation in treatment of insulin resistance, IGT and/or type II diabetes.